isValidJSValue does not always return an array

I'm not sure if this is an expected behaviour of the method, as the docs say that it returns an array of errors. Here's a failing test case in mocha:

```js
describe('isValidJSValue', () => {
  it('is returning an array of errors', () => {
    expect(isValidJSValue('a', GraphQLInt)).to.be.an.instanceof(Array);
  });
});
```